In medical radiography, an image of a patient's tissue and bone structure is produced by exposing the patient to X-radiation and recording the pattern of penetrating X-radiation using a radiographic element containing at least one radiation sensitive silver halide emulsion layer coated on a transparent (usually blue-tinted) support. Differences in X-radiation absorption by various body tissue, i.e. subject contrast, result in image-wise differences in silver density in the developed silver image, i.e. radiographic contrast. X-radiation can be directly recorded by the aforementioned emulsion layer where only limited areas of exposure are required, as in imaging of body extremities. However, a more efficient approach which greatly reduces X-radiation exposures, is to employ an imaging screen in combination with the radiographic element. The imaging screen, commonly referred to as an intensifying screen, absorbs X-radiation and emits longer wavelength electromagnetic radiation which silver halide emulsions more readily absorb. Another technique for reducing patient exposure is to coat two silver halide layers on opposite sides of the film support to form a "double-coated" radiographic element. Diagnostic needs can be satisfied at the lowest patient X-radiation exposure levels by employing a double-coated radiographic element in combination with two imaging screens.
A more recently developed approach for radiographic diagnostic imaging is to employ a second type of imaging screen, commonly referred to as a storage phosphor screen, instead of one or two imaging screens of the first type (i.e., intensifying screens) and a radiographic element. This imaging approach was first proposed by Luckey U.S. Pat. No. 3,859,527 (reissued as Re. 31,847). Storage phosphor screens can be essentially similar in construction to X-ray intensifying screens, differing primarily in the composition of the phosphor selected. Storage phosphor screens are imagewise exposed to X-radiation that is again absorbed by the phosphor particles. Although the phosphor may promptly fluoresce to some degree, most of the absorbed X-radiation energy is retained in the phosphor particles. When stimulated with longer wavelength radiation the screen emits in a third wavelength region of the spectrum. Typically an X-ray imaging screen of the storage phosphor type is used alone for imaging. After imagewise exposure it is typically stimulated to emit by scanning, and the emission pattern is stored in computer memory. The image can be viewed as a video display, but more typically a hard copy of the image pattern is produced for careful study by transferring the image information from computer memory to a silver halide radiographic element via laser exposure.
In both approaches image capture by an imaging screen are similar and usually the image that is ultimately produced for close inspection is a silver image produced in a radiographic element. Hence the imaging limitations of both approaches are similar.
An imagewise exposed and processed radiographic element is primarily intended for viewing by transmitted light. In a typical situation a medical radiologist studies the silver image with the radiographic element mounted on a light box, a white translucent illumination source. An accurate diagnosis requires that the silver image accurately differentiate between diseased and healthy tissue which may sometimes be distinguished by differences in silver density, which result from differences in subject contrast. Unfortunately, such density differences are very difficult to detect in soft tissue since such soft tissue anomalies frequently fail to provide sufficient subject contrast such that radiographic contrast within the silver image is sufficient to provide for an accurate diagnosis.
Low subject contrast is a particular problem in neonatal radiography where the bone structure of a newborn infant is not fully developed and it is important to diagnose problems that may occur in areas such as the heart, lungs and intestines, all of which are soft tissue. An example of one such important diagnosis is the detection of hyaline membrane disease that manifests itself as a fine "ground glass" pattern in the lungs. In the absence of sufficient subject contrast, such a pattern can be easily mistaken for and/or obscured by radiographic noise in the film screen system which provides the radiographic silver image. Furthermore, this problem cannot be solved by simply increasing radiographic film contrast because increasing such film contrast increases radiographic noise or mottle which obscures the pattern. Diagnosis of this disease in the very first days of life in a neonate is critical as this condition is life-threatening. Accordingly, an accurate diagnosis of hyaline membrane disease, as well as other diseases, and treatment at an early stage is essential.
The effect on filter efficiency, subject contrast and exposure level or dosage by modifying a diagnostic X-radiation beam with a filter using a water phantom and a detector consisting of a pair of intensifying screens immediately behind the water phantom, is the subject of a computer simulation study based on photon transport calculations reported in an article by Raymond Carrier and Rene Beique, "Analogous Filters for Beam Shaping in Diagnostic Radiology", Phys. Med. Biol., 1992, Vol. 37, No. 6, 1313-1320. Printed in the U.K. The article states that the study was limited to filters containing materials having odd-numbered atomic numbers and the conclusions are based upon a calculation involving a large number of such filter materials. No experimental data is provided in the article. Also, there is no indication whether the filter materials were metals or nonmetals or both. However, the report presents these conclusions: (1) "Unpredictable behavior was observed with higher atomic number filter materials in the range of 40 to 70: a small change in any of the parameters changed the efficiency, the contrast and the integral dose. Occasionally the contrast increased within this range of atomic number, but invariably in these cases, the efficiency was very low and the integral dose was high." and (2) "Even with the extensive combinations of parameters used, no magic filter was found which would produce increased contrasts or a decreased integral dose, while maintaining efficiency similar to that of aluminum. Filters of some atomic numbers produced increased contrast, but had negligible efficiency.". The report also sets forth a curve, FIG. 1, which depicts the semi-log plot of the thickness required (kg m.sup.-2) to produce analogous filters normalized to the thickness of aluminum. This curve shows a hiatus for filter materials having atomic numbers between approximately 45 and 55 which indicates that no suitable filter was found for materials having atomic numbers within this range or that the efficiency of such materials would be insignificant. In light of the teachings of the Carrier et al., article, it is clear that the use of X-radiation source filters to improve subject contrast is both empirical and highly unpredictable. In addition, the data reported in the article, particularly, the aforementioned curve, suggests that filter materials having atomic numbers within the range of 40 to 70 would be unsuitable for this purpose. Also, there is no description of any specific filter material or its use in the system described.
This invention addresses the problem of enhancing the quality of a radiographic image by increasing subject contrast in soft tissue anomalies, particularly soft tissue anomalies which normally exhibit subject contrasts of less than 10 percent. It is evident that it would be desirable to provide an image-forming combination which achieves the aforementioned increased subject contrast without exposing the patient to unacceptable X-radiation exposure levels. This invention meets this objective.